In the last decade, substantial progress has been made in understanding the molecular mechanisms involved in initial host responses to viral infections, and how viral recognition leads to the innate responses that ultimately shape the adaptive immune response. Viruses, including herpes simplex virus (HSV) types 1 and 2, trigger toll-like receptors (TLRs) that elicit cytokine and chemokine production. In turn, this can create local resistance and modulate T- and B-cell-mediated immunity. TLR activation by HSV-produced molecules (or other synthetic agonists) leads to the remodelling of draining lymph nodes. This enhances the screening of naive T-cells, from which antigen-specific lymphocytes can be selected and expanded. The innate response thereby serves to direct a timely and effective acquired immune response, through the initial TLR recognition of viral pathogen-associated molecular patterns that can limit or possibly exacerbate viral pathogenesis. Recently, these findings have been exploited by strategies that utilize synthetic TLR agonists as prophylactic or therapeutic devices. Such devices prime innate immune responses, enhancing host resistance to viral infections, including experimental HSV infections.
The development of a vaccine to prevent norovirus infections has been focused on immunization at a mucosal surface, but has been limited by the low immunogenicity of self-assembling Norwalk virus-like particles (NV VLPs) delivered enterically or at nasal surfaces. Nasal immunization, which offers the advantage of ease of immunization, faces obstacles imposed by the normal process of mucociliary clearance, which limits residence time of applied antigens. Herein, we describe the use of a dry powder formulation (GelVac) of an inert in situ gelling polysaccharide (GelSite) extracted from Aloe vera for nasal delivery of NV VLP antigen. Powder formulations, with or without NV VLP antigen, were similar in structure in dry form or when rehydrated in simulated nasal fluids. Immunogenicity of the dry powder VLP formulation was compared to equivalent antigen/adjuvant liquid formulations in animals. For the GelVac powder, we observed superior NV-specific serum and mucosal (aerodigestive and reproductive tracts) antibody responses relative to liquid formulations. Incorporation of the TLR7 agonist gardiquimod in dry powder formulations did not enhance antibody responses, although its inclusion in liquid formulations did enhance VLP immunogenicity irrespective of the presence or absence of GelSite. We interpret these data as showing that GelSite-based dry powder formulations (1) stabilize the immunogenic structural properties of VLPs and (2) induce systemic and mucosal antibody titers which are equal or greater than those achieved by VLPs plus adjuvant in a liquid formulation. We conclude that in situ gelation of the GelVac dry powder formulation at nasal mucosal surfaces delays mucociliary clearance and thereby prolongs VLP antigen exposure to immune effector sites.
